An X-ray CT system is a system that irradiates X-ray beams that are subjected to a rotating operation toward a subject, and forms tomographic images, etc. based on information obtained by detecting X-rays transmitted through the subject. In recent years, as this X-ray CT system, a system provided with a two-dimensional detector has been suggested. A two-dimensional detector has a plurality of rows of detection parts in the rotational axis direction (body axial direction of the subject) of a source that generates beams. Each row of the detection part consists of detection elements aligned in a linear fashion in the rotational direction of the source that generates beams.
Providing a plurality of rows of detection parts in the rotational direction in this way is referred to as multiple detection rows.
In this X-ray CT system equipped with a two-dimensional detector, data over an extensive region of a subject can be acquired by scanning through one rotation. Therefore, in the X-ray CT system using a two-dimensional detector, the number of rotations is small, which is an advantage when attempting to streamline imaging.
By using this X-ray CT system equipped with a two-dimensional detector, multi-slice scanning, volume scanning, dynamic volume scanning, helical scanning in multiple rows, etc., can be performed. Volume scanning is a scanning method in which a whole body is scanned and the data is stored as volume data, and filming, image transfer, etc. can be subsequently performed with respect to the volume data. In addition, the volume data can be divided into a plurality of areas by site to individually designate reconstruction conditions. For example, it is possible to scan from the shoulder to the top of the head at once, and reconstruct an image with a thickness of 10 mm for the top of the head, a thickness of 5 mm for the skull base, and a thickness of 1 mm for the neck. Helical scanning is a method in which X-rays are helically irradiated by rotating a source that generates X-rays and a two-dimensional detector while moving a subject in the body axial direction in order to obtain data. This may cause the source of generating X-rays and the two-dimensional detector to move.
Moreover, when scanning of a region of interest is conventionally performed by performing volume scanning once or a plurality of times, scanning is performed after the number of rows to be used for scanning and the number of times the scan is to be performed are determined such that X-rays are not irradiated to unnecessary regions within the scanning range, with the same number of scanning rows and with the smallest possible number of times the scan is performed (refer to Japanese Unexamined Patent Application Publication No. 2001-59872).
In addition, the subject is exposed to X-ray irradiation, but when this exposure dose is high, the subject is adversely affected.
Therefore, techniques for reducing exposure doses have also been suggested. These techniques include, for example, a Real-EC that calculates the minimum mA with which image display of all slices is possible with the same S/N based on a positioning image, and a Volume EC that calculates the minimum mA from two directions and performs current control such that image display of all slices is possible on the same image from a positioning image, etc.
However, with the conventional method described in Patent Document 1, because a series of scans is performed with the number of rows determined once, the same intensity of X-rays is irradiated to the range for the row width. Therefore, if X-ray irradiance is determined for the purpose of generating an image with good quality, even when there is a part in which exposure to irradiation should be avoided, the X-ray irradiance cannot be reduced only for that part, resulting in higher X-ray irradiance. In addition, when it is intended to reduce irradiation to a part in which exposure to irradiation should be avoided, the X-ray irradiance is also reduced for the surrounding areas of the part in which exposure to irradiation should be avoided, resulting in lower quality of images.
Moreover, although there are techniques to reduce exposure to irradiation such as the Real-EC and the Volume EC as described above, these methods only calculate the dosage of X-rays required to obtain the same quality of images in an entire scanning region, and do not consider parts in which exposure to irradiation should be avoided, such as highly X-ray sensitive parts. In addition, in these methods, the number of rows during scanning is also fixed. Therefore, in these methods as well, the wider the width of the detector in the body axial direction becomes due to multirows, etc., the wider the range in which X-rays are irradiated in one rotation becomes, resulting in scanning sites with different structures in the subject simultaneously; hence, it is difficult to suppress irradiance of X-rays only for parts in which exposure to irradiation should be avoided.
Moreover, with the conventional techniques, in order to perform X-ray irradiation with low X-ray irradiance on regions in which exposure to irradiation should be avoided, it has been necessary to manually set the irradiation fields and the number of rows of detection elements across the entire range of a region of interest, respectively, and doing so is complicated.